Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to a method for measuring the flow rate of a fluid. In particular, it relates to a flow measurement method in which a portion of the fluid is periodically given a motion in addition to that of the fluid flow, and in which that additional motion can be sensed to determine flow rate.
2. Background Information
For reference purposes, there are a number of different types of flow sensors which use acoustic methods to sense the flow rate of a fluid. One major type is the ultrasonic transit time flow sensor, also known as the xe2x80x9ctime of flightxe2x80x9d ultrasonic flow sensor, which detects the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the flowing fluid and which processes the information to derive a fluid flow rate. Another major type is the Doppler type which detects the difference in frequency between transmitted acoustic signals and those reflected by acoustically reflective entities in the fluid. Both of these methods utilize relatively high frequencies for the acoustic transmissions (typically hundreds of kilohertz to several megahertz). Both methods are somewhat limited in application as they require sophisticated construction and supporting electronics which tend to make them large and expensive, and can produce substantial measurement errors under less than ideal conditions. Vortex shedding flow sensors detect relatively small easily corrupted signals and arc limited in rangeability. Other methods, such as those described in my U.S. Pat. Nos. 4,348,906 and 4,462,264, use relatively low frequency ultrasonic and sonic transmissions to sense the rotation of a mechanical element, such as a turbine rotor, which is responsive to flow rate. Their big disadvantage is, of course, the reliance on a moving mechanical element which is often thought to be less reliable than other methods which are more electronically oriented and do not have any recognizably moving parts. Other methods also cited for reference purposes which rely on mechanical means for modulating the fluid flow are described in my U.S. Pat. Nos. 5,948,978, 6,023,969 and 5,390,541.
It is therefore an object of this invention to produce a flow sensor capable of good measurement precision which is relatively easy to manufacture, requires simple supporting electronics, is adaptable to a wide range of installations and does not rely on the rotation of a mechanical element for its basic measurement of flow rate. A preferred inertial flow sensor of the present invention satisfies this and other objectives. The preferred flow sensor operates by setting into circular motion a small volume of the fluid being sensed and then detecting the rate of travel of that rotating fluid volume due to the fluid flow.
In one of the preferred embodiments of the present invention, a transmitting transducer is energized by electric pulses at a low frequency (typically several hertz to several kilohertz) to provide corresponding pulses of movement in the fluid. The fluid movement displaces a volume of the fluid in one direction and the motion of that fluid, when combined with the surrounding fluid filling the void, produces a fluid rotation which is sustained for a sufficient period of time because of inertia, to be detected at a later time as an acoustic signal. A known distance downstream of the transmitting transducer, a receiving transducer detects the rotating volume of fluid. From the distance between the transmitting and receiving transducers and the time interval between their transmitted and detected signals, the fluid flow rate can be determined. The transmitting and receiving transducers can interchange their functions so that the difference in the time intervals is used to determine fluid flow rate. The transmitting transducer may also be energized with single pulses, a burst of pulses, or a continuous wave of pulses so that various methods, including phase detectors, may be used to determine the time difference between the received and transmitted, or reference signal, for providing a fluid flow rate signal.
In another preferred embodiment of the invention, a transmitting transducer is energized and rotating volumes of fluid are generated as in the first preferred embodiment. However two receiving transducers, one located upstream and the other located downstream of the transmitting transducer, are used to detect the rotating volumes of fluid. The difference in time between the reception of the signals is a measure of fluid flow rate.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.